Natural gas is considered to be one of the most clean and ecological energy for the future and it is considered to be an area of competitiveness between various oil companies. Global demand for clean energy is increasing and especially the clean hydrogen is necessary for the use in the proton exchange membrane fuel cell. Methane is the least reactive and most abandoned natural gas on the earth. So, selective oxidative functionalization of methane is of great importance due to the growing energy demand and the depletion of fossil fuel. Methane, the most abandoned and predominant component of the natural gas is forecasted to outlast oil within 60 years. Therefore researchers are concentrating on the utilization of methane by its activation because of its plentiful abundance in many locations around the globe. The current mean projection of remaining recoverable resources of natural gas is 16,200 Trillion cubic feet (Tcf), 150 times than the current annual global gas consumption. In recent time the use of natural gas as a feedstock to synthesize chemicals or fuels is not economical because of the costly storage process and transportation system from the remote areas of the globe where it is mostly available. Particularly in recent 20 years, methods to enhance value of the natural gas methane have been investigated either by synthesizing more valuable chemicals or more easily transportable fuels. But the yields are found to be too low and because they are more reactive than methane itself and is unable to compete with the oil. Oxidative methane coupling to ethane gives maximum achievable amount of yield around 30% because there is an inherent limit and since an important part of the reaction goes through gas phase reaction kinetics. Another process is continuous direct conversion of methane to methanol or formaldehyde and the maximum yields so far achieved is 8% and 4% respectively. A recently described batch process gives >50% of methanol but it is not an ideal process because of the use of a mercury catalyst and sulfuric acid which produces sulfur dioxide and it is need to be converted back to sulfuric acid. Industrial processes for the production of hydrogen cyanide by the reaction of methane and ammonia or ammonia, oxygen and ethane by pyrolysis are available but both the processes have their drawbacks due to the high temperature requirement (more than 1027° C.). Therefore at this time the most useful economical path to utilize methane is the production of more valuable chemicals through synthesis gas generation. Now several synthesis gas production processes are available based on the purpose of industrial applications. Synthesis gas can be produced by steam reforming of methane, CO2 reforming of methane, partial oxidation of methane and decomposition of methanol (mainly used in hydrogen production in fuel cells because methanol is high in energy density and easy to transport). Industrially methanol is synthesized from syngas generated from coal or natural gas. The production of synthesis can be done through following processes, (1) steam reforming of methane, (2) CO2 reforming of methane, and (3) partial oxidation of methane. Till date steam reforming is the only large scale syngas production process. Steam reforming is highly endothermic and the current industrial catalysts are used in Nickel based. However nickel promotes carbon formation which deactivates the catalyst and reactor plugging. Overcoming this problem creates a new problem, the higher H2/CO ratio where low ratios are desirable for industrial downstream processes. Therefore an alternative process can be synthesis gas formation by partial oxidation of methane where the H2/CO ratio is perfect for the downstream processes, particularly for methanol synthesis.
Partial oxidation of methane has thermodynamic advantages over steam reforming.    (1) Partial oxidation of methane is lightly exothermic while steam reforming is highly endothermic. So partial oxidation is more economical and it can also be combined with other endothermic processes, such as steam reforming or dry reforming of methane to make this process more energy efficient.    (2) The H2/CO ratio produced in stoichiometric partial oxidation is around 2 which are perfect for the industrial downstream processes, particularly for methanol synthesis. Therefore it avoids the removal of valuable hydrogen, which is produced in steam reforming process.    (3) The synthesis gas mixtures obtained by partial oxidation of methane contain very low amount of carbon dioxide content.    (4) Partial oxidation of methane avoids the need for large amount of superheated steam which is required in steam reforming. But a costly oxygen separation plant may require where nitrogen is undesirable in high pressure downstream processes.
Because of the above depicted reasons partial oxidation of methane is likely to be more important for the synthesis gas production in the recent future. The papers detailing the catalytic partial oxidation of methane to synthesis gas shows that high yields of synthesis gas can only obtained above 850° C., below this temperature non equilibrium product distribution is obtained. Thermodynamic calculation shows that higher temperature is favorable for partial oxidation of methane to produce very high H2 and CO selectivity. The conventional supported nickel catalyst used for methane reforming are very active for carbon formation leads to rapid deactivation of catalyst, while coke-resistance alternatives (Rh, Ru, Pt etc) are bounded by its availability and high cost. So economic boundary conditions dictate the use of Ni based catalysts. There are reports on partial oxidation of methane over different solid catalyst but to the best of our knowledge there is no reference for the use of Ni—CeO2 catalyst for this purpose, where the catalyst is stable till 100 h without any deactivation.
Reference may be made to article in the Applied Catalysis, 2011, 401, 170-175 by Z. A. Fedorova et al. where they reports Ni catalyst support on the basis MgO catalyst for partial oxidation of methane. At optimized condition with 4.5% Ni supported on Ni—MgO shows 75% methane conversion at 800° C. with a GHSV of 127000 ml g−1 h−1, but after 24 h time on stream the conversion drastically goes down to 55%.
Reference may be made to article in the Fuel, 2012, 97, 630-637 by E. M. Assaf and his group reported partial oxidation of methane over NiO—MgO—ZrO2 catalyst with 2:1 methane to O2 at 750° C. for 6 h. 40 mol % MgO showed highest conversion of ≧90%, but the catalyst shows low carbon resistance up to 6 h of time on stream with the H2/CO ration of 1.6.
Reference may be made to U.S. Pat. No. US6,254,807B1 by Schmidt et al. on “control of H2 and CO production in partial oxidation process” where they use at least one transition metal (preferably Ni) monolith catalyst under partial oxidation condition. In in optimized condition with monolith catalyst of 50% porosity with GHSV between 60000 to 3000000 h−1 to achieve a methane conversion of 70% with Ni as a catalyst where it goes to 80% with Rh catalyst. Furthermore, the catalyst shows coke resistivity up to 40 h on time on stream.
Reference may be made to U.S. Pat. No. 6,402,989 B1 by A. M. Galgney, where his invention relates to a catalyst and process using promoted (at least one from the group consisting Mn, Mo, W, Sn, Re, Bi, In, P etc.) nickel based catalyst supported on MgO. The catalyst contain 1 to 50 wt % of Ni with 0.1 to 10 w % of one promoted where with Mn promoted Ni catalyst shows 100% conversion of methane at 100000 GHSV ml h−1 g−1 730° C. at a pressure about 850 to 3000 kPa. But the main drawback of the process is to achieve high conversion high pressure is also required with high temperature whereas implementing such pressure may leads to phase sintering.
Reference may be made to article in the Phys. Chem. Chem. Phys., 2002, 4, 4549-4554 in which M. L. Green and his group reported the production of syngas by partial oxidation of methane over molybdenum carbide. 96% of methane conversion was achieved at 900° C. and 8 bar pressure with CH4/O2=2.03 and GHSV of 2000 h−1. But the catalyst have drawback such as high cost of catalyst, considerably low GHSV, high operational pressure etc.
Reference may also be made to article in the Energy & Fuels 2003, 17, 474-481 in which R. G. Mallinson et al. reported synthesis gas production in an AC electric gas discharge of methane and air mixtures at room temperature and ambient pressure. In this report methane with oxygen and helium; CH4/O2/He Molar Ratio, 3:1:3.8; Overall Flow Rate, 100 cm3/min; Gap Distance, 10 mm; Residence Time, 0.23 s with applied voltage 300 Hz only 18% methane conversion was observed.
Reference may be made to article in the Nature, 1990, 344, 319-321 by P. D. F. Vernon and his group reported to carry partial oxidation of methane at a temperature of 775° C. using lanthanum ruthenium oxide catalyst. In reaction with the N2, methane conversion is around 90% with the selectivity of in the range of 94-99%, were found for all the rare earth oxide support with ruthenium catalyst after an induction period lasting approximately 30 minute. But the main drawback of the catalyst is it required heavy investment as the rare earth metal and ruthenium is less abundant in earth crust.
Reference may be made to article in the Indian Journal of Chemistry, Vol. 53A, April-May 2014 pp. 467-471 by Rajaram Bal et.al. where, the authors reported Ni—CeO2 catalyst for the partial oxidation of methane. At optimized condition with 5% Ni—CeO2 catalyst shows 86.1% of methane conversion at 800° C. The main drawback of the catalyst is its time on stability which is 30 h. and the catalyst shows activity at minimum 500° C.
The drawback of the processes reported so far is that although they exhibit sufficiently high conversions of methane for high selectivity of syngas of H2/CO ratio almost 2 but the rapid formation of coke due to the local heat generation causes deactivation of catalyst. To overcome the deactivation many researchers used novel metals such as Pt, Ru, Rh etc but the rising cost and relatively poor availability desiccates the use of those catalyst in industrial purpose. On this economic boundation, Ni based catalyst will be the holy grail for the utilization of methane to future fuel in coming future. There is, therefore, an evident necessity for further improvements in the Ni based catalyst and process for the partial oxidation of methane.